Anodised helical tape waveguide for modulating arrangements



ANODISED HELICAL TAPE WAVEGUIDE FOR MODULATING ARRANGEMENTS INVENTOR Bfluid/Mam 5mm xIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII.lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllI- J. R. LOHOAR Sept. 11, 1962 Filed April 29, 1960 ATTQRNEYS UnitedStates Patent 3,054,075 AN ODISED HELICAL TAPE WAVEGUIDE FOR MODULATINGARRANGEMENTS John Ronald Lohoar, Steveuage, England, assignor toMarconis Wireless Telegraph Company Limited, London, England, a Britishcompany Filed Apr. 29, 1960, Ser. No. 25,573 Claims priority,application Great Britain Aug. 19, 1959 Claims. (Cl. 333-96) Thisinvention relates to wave energy modulating arrangements and morespecifically to modulating arrangements of the kind in which very highfrequency wave energy is modulated by propagating it along a waveguidein which is a ferrite or similar body and subjecting said body to amodulated or varied magnetic field produced by a coil system outside theguide.

The object of the invention is to provide improved, simple and efficientmodulating arrangements of the kind referred to wherein, despite thatthe coil system is outside the waveguide, there is minimum losses due tothe interposition of the walls of the guide in the path of the magneticflux and minimum interference with the proper functioning of thewaveguide as such.

It is common practice to effect modulation of a high frequencyelectro-magnetic wave transmitted down a waveguide by applying amagnetic field which is varied in accordance with modulation to vary themagnetisation of a ferrite body inserted in the waveguide, themodulating magnetic field being generated by a coil system outside theguide. One of the diificulties met with in modulators of this nature isthat, since the varying magnetic field has to pass through the guidewalls to reach the ferrite body, it induces substantial eddy currents insaid walls, especially if the modulating frequencies are relativelyhigh. These eddy currents of course constitute loss and tend in effectto short circuit the magnetic field and oppose proper and efiicientmodulation. Various expedients have been proposed to meet thisdifficulty but it is by no means easy to do so without interfering withthe proper functioning of the waveguide as such, and without resultingin a structure which is mechanically unsatisfactory or unduly expensiveor incapable of handling high power. For example, it has been proposedto use a guide in the form of a very thin film of deposited silver oraluminium on the inside of an insulating tube, but this constructionwill not handle high power satisfactorily and is, moreover, expensiveand mechanically weak. It has also been proposed to slot the waveguidewith a slot running parallel to the axis, but here again such a slottends to distort the field, its provision results in an asymmetricalwaveguide and leakage of high frequency (microwave) energy tends tooccur through the slot. Another proposal which has been made is toprovide the waveguide with a helical slot, the slotted waveguide beingformed by electro-forming it on a special mandrel and then using thecoil, which the slotted guide in fact constitutes, as the source of themod ulating magnetic field. This expedient avoids the difiiculty abovementioned by in fact dispensing with the external modulating fieldsource altogether (since the guide itself is now the field source) butthe construction is expensive and again there is a tendency to microwaveenergy loss through the helical slot. There is also an added limitationin design parameters because of the fact that the magnetisation coil andthe guide are now one and the same.

According to this invention an arrangement of the kind referred to formodulating very high frequency electromagnetic Wave energy comprises alength of waveguide constituted by a helically wound metallic tape withthe edges of the turns substantially in physical contact with oneanother but insulated from one another, a ferrite body ICC Within saidlength of waveguide and a winding external to said length of waveguideand arranged and adapted to provide a varying magnetic field throughsaid body to modulate, in accordance with the variations of said field,high frequency wave energy transmitted through said length of guide.

Preferably the tape is aluminium tape anodised to provide the insulationbetween the turns.

In order to reduce discontinuities between the helically wound length ofguide and the adjoining ends of waveguides between which the length isinserted to form a run of guide, it is most desirable, if not essential,that the ends of the helically wound length be finished flat, i.e. thatthey terminate in planes at right angles to the axis. One way of doingthis is to fit each end of the helically wound length with a cylindricalcollar which is flat at one end and shaped with a step at the other tomate with the end of the helix proper, the tape of which is cut oifsquare to the length of the tape. Another way is to solder together thelast few turns of the helical guide to permit it to be turned offsquare, i.e. flat.

Preferably also the outside of said length of guide is covered with athickness of comparatively rigid insulating material, e.g. resin, andthe winding is positioned on the outside of this insulating material. Inone construction, the winding is a single layer winding of turns ofconductive tape wound directly on the insulating material.

The ends of the length of waveguide may be fitted with flanges whichmust be so arranged that, when said length is inserted in a run ofguide, it is insulated at least at one end from the adjacent guides.Accordingly, if a flange on the end of the helically wound length is tobe in metal to metal contact with a flange at the end of an adjacentlength of guide, the former flange should be insulated from the helixand a thin insulating shim is inserted to avoid contact between the endof the helix and the adjacent guide.

The invention is illustrated in the accompanying drawing which is asimplified sectional view of a preferred embodiment of the invention asapplied to a frequency or phase modulator of microwave energy. Althoughthe invention is not limited to its use at any particular band offrequencies, it is very advantageous for phase or frequency modulationof microwave energy'at comparatively high frequencies and theillustrated embodiment is suitable for use for phase or frequencymodulating microwave energy of a frequency of the order of 10,000 mc./s.by a modulating frequency of the order of l mc./ s. Practical principaldimensions for the illustrated embodiment when intended for thefrequencies stated are: about9 A2" overall length and about 0.81" forthe inside diameter of the guide, the guide being wound from anodisedaluminium tape about A of an inch wide and about of an inch thick.

Referring to the drawing, a circularly sectioned guide 1 is made fromanodised aluminium tape which is wound in helical form with the edges ofthe turns in contact with one another as indicated by the parallel lines1 shown at the ends of the guide length. It will be understood that thedistance between any two adjacent parallel lines 1 represents the widthof the tape. The anodising of the aluminium provides insulation so thatalthough the tape edges are physically in contact with one another andthe finished guide has no gaps or substantially no gaps leading to theexterior, the turns are effectively insulated each from the next. Theends of the wound guide are finished off by flanges which serve forcoupling the length of guide to other guide lengths (not shown). Theseflanges should be insulated from the guide proper and may be fittedthereto in any convenient manner. The most practical way of making theglide consists in winding the tape on to a mandrel so .that no gapsoccur in the winding; clamping it at the ends; and then, after trimmingthe ends if necessary, fitting the flanges in position with suitableinsulation between them-and the guide proper. The clamps may then beremoved and the mandrel punched out.

In the illustrated embodiment the waveguide with its end flanges, whichare marked 2, is potted with resin or some other suitable insulatingcompound so that the guide is externally coated to provide it withadditional mechanical: strength and rigidity by a comparatively thicklayer of resin or other insulating compound 3. On this layer is woundthe modulating magnetizing coil which is conventionally represented at 4and may conveniently consist in practice of a few turns of copper tapewound directlyon to the insulation 3. The coil 4 extends at least overthe length of a ferrite insert which is shown as of conventional form at5, i.e. it is a ferrite rod with pointed ends for avoiding sharpimpedance discontinuities. The ferrite rod 5 is supported in position inthe guide by suitable insulating supports 6, preferably aper-tured asindicated.

, An experimental arrangement as illustrated using ano dised aluminiumtape 0.1" x 0.032 has been successfully used at the frequencieshereinbefore mentioned to handle 500 watts C.W. power.

I claim:

1. A wave energy modulating arrangement in which very high frequencyenergy is modulated comprising a length of waveguide, a ferrite bodylocated in said waveguide, a magnetic coil positioned outside saidlength of waveguide arranged and adapted to subject said body to amodulated magnetic field, said waveguide being of circular cross-sectionand comprising ahelically wound aluminium tape the turns of which aresubstantially in physical contact with one another, said tape beinganodised to provide insulation between the turns whereby energytransmitted through said length of Waveguide is modulated by the varyingmagnetic field through said body.

2. An arrangement as claimed in claim 1 wherein the ends of thehelically Wound Waveguide terminate in planes at right angles to theguide axis and wherein the last few turns of said tape are solderedtogether.

3. An arrangement as claimed in claim 1 wherein the said guide iscovered with a layer of insulating material and wherein said coil ispositioned on the outside of said insulating material.

4. An arrangement as claimed in claim 1 wherein each end of thehelically wound length is fitted with a cylin drical collar which isflat at one end and shaped with a step at the other to mate with the endof the helix proper, the tape of which is cut oif square to the lengthof the tape.

5. An arrangement as claimed in claim 1 in which said waveguide lengthis fitted at least at one end with a flange insulated from the helicalwinding.

References Cited in the file of this patent UNITED STATES PATENTS2,802,134 Fox Aug. 6, 1957 FOREIGN PATENTS 1,031,381 Germany June 4,1958 OTHER REFERENCES Brown, Proceedings of the IRE, April 1958, pages722-726.

